The present invention relates to X ray generation systems and, more particularly to such systems which convert an electrical input to X rays using the phenomenon of a gas jet z-pinch.
Presently integrated circuits are manufactured using ultraviolet light lithographic systems. In such systems, the circuit pattern is determined, and a mask is prepared in accordance with the pattern. The mask is a thin plate having transparent and opaque sections according to the pattern. Upon illumination of the mask with the ultraviolet light, an image is projected onto a silicon wafer having a photoresist coating. A relief pattern replicating the mask is provided upon chemical treatment to remove either the exposed or unexposed resist (depending upon the type of resist process employed). Subsequent etching, doping or metallization steps impart the desired electrical characteristics to the wafer, and the remaining resist is removed, resulting in the formation of one level of an integrated circuit.
Commercially available ultraviolet light lithography systems offer pattern resolution on the order of 1.5-2 microns. Such a level of resolution is adequate for the production of integrated circuits such as a 256K random-access memory; however, for still larger scale integrated circuitry, lithographic systems providing submicron pattern resolution are needed, if the products are to be kept small. An X ray lithography system incorporating a pulsed plasma source provides the finer resolution desired. The system converts an electrical input to X rays using the phenomenon of a gas jet z-pinch. In this method of X ray generation, a burst of a gas (such as nitrogen, krypton or argon) is expanded using a nozzle, in concert with the fast discharge of a capacitor bank through the expanding gas. A high current discharge generates an intense magnetic field which radially compresses the plasma, a z-pinch. The result is a dense, high temperature plasma which is a very intense source of desirable X rays with comparatively long wave lengths and, hence, low penetrating power (commonly known as soft X rays). Heretofore, gas jet z-pinch devices were used for thermonuclear fusion experimentation. They were used for single-shot, as opposed to repetitive applications.
Commercial applications of repetitively pulsed plasma X ray sources for areas such as X ray lithography require long lived, highly reliable systems. Prior high power, high voltage pulsed devices have used spark gap switches employing either a single rail gap switch for generating a high current electrical discharge or an array of such switches. Unfortunately, the switches have limited life less than 1 million shots) when operated with high currents and are, therefore, not practical for low maintenance, long lived, repetitively pulsed systems. Another disadvantge of these switches is that they produce strong acoustic vibrations due to switch arcing. These vibrations adversely affect the operation of systems components, such as X ray aligners, which require very stable conditions for high accuracy positioning. Another disadvantage of spark gap switches is that they generate excessive electrical noise which can induce currents disruptive of microprocessors and electronics which may be located in close proximity.
A saturable inductor switch has been proposed for compressing the width and sharpening the rise time of high voltage pulses to an electrical discharge gas laser. For further information regarding the structure and operation of such a switch, reference may be made to U.S. Pat. No. 4,275,317.